Inset Boundary and Mirror only?
Good question. You've picked up what looks like an anomaly, unless someone comes along to correct us.

In all the cases in the main square below, the right two faces are selected before 'Inset' is called.

In the top row, 'Individual' is not checked. The two faces are considered a region. The inset is made on all edges at the boundary of the selection, ignoring edges at the internal boundaries between faces.

In the top left case, 'boundary' is checked, permitting the inset at the boundaries between the region and the outside world. In the right case it is not, and the external boundaries are not inset.

In the second row, 'Individual' is checked. An inset is made at all internal boundaries as well as external, when 'Boundary' is checked, on the left.

However when 'Boundary' is not checked, I would expect the inset to be made at internal, but not external boundaries, producing a result like the last in the bottom row. But it doesn't.

The only way I could get the expected result on the far right was to select each face in turn, with 'Individual' not checked, treating each face as a 1-face region, and inset each region individually, with 'Boundary' unchecked.



In short: the  'Individual' setting does not respect the distinction between internal and external boundaries made in the 'Boundary' setting.